Supplementary oxygen is of critical importance in many situations, including use (1) as a first aid measure, (2) treatment of chronically ill patients, and (3) prevention of hypoxemia (lack of oxygen in arterial blood) at high altitudes. The therapeutic effects of supplementary O.sub.2 include: elimination of nitrogen bubbles in tissue or blood vessels, oxygenation of plasma to increase physically dissolved oxygen, reduction of tissue edema, and increase O.sub.2 saturation of hemoglobin. In each of these examples, the user of a supplementary oxygen delivery system desires to maintain a certain inspired O.sub.2 percentage for a given duration of time. However, in some situations such as in remote locations, the supply of available oxygen is limited. This makes the efficiency of the delivery system an important factor. Also, the percentage of oxygen required may differ according to the situation. For example, in many emergency applications as close to 100% inspired O.sub.2 as possible is desired.
On the other hand, for high altitude applications, a percentage only high enough to achieve 90% arterial O.sub.2 saturation may be sufficient. Therefore, control of the percentage of oxygen delivered to an individual is significant in making a system versatile as well as efficient. Applicants' invention provides a system with improved efficiency for delivering a given percentage of O.sub.2 for the longest duration.